1. Field of the Invention
The present invention relates to a polishing slurry for aluminum-based metal, and in particular, to a polishing slurry for aluminum-based metal which is suited for use in the formation of a damascene wiring to be employed in a DRAM, an FeRAM or a high-speed logic LSI, and to the method of manufacturing a semiconductor device by making use of this polishing slurry.
2. Description of the Related Art
In recent years, a buried wiring (damascene wiring) processing technique has been intensively studied and developed in order to simplify the manufacturing steps and to improve the yield and reliability in the back-end process in the semiconductor super-LSI manufacturing technique. In particular, the CMP (Chemical Mechanical Polishing) technique is indispensable in the process of forming a damascene wiring.
At present, a copper damascene wiring technique where copper is employed as a wiring metal is mainly employed in the back-end process of a high-speed logic device.
On the other hand, in the back-end process of a memory device represented by DPAM, the damascene wiring technique where aluminum or tungsten is employed as a wiring metal is employed in view of low processing cost. Among them, a damascene wiring using aluminum (aluminum damascene wiring) is considered as being most promising in view of the fact that aluminum is lowest in electric resistance next to copper.
The manufacturing process of aluminum damascene wiring is mainly constituted by a step of forming a wiring groove in an interlayer insulating film, a step of forming an aluminum film (burying step), and a step of performing CMP of aluminum. It is required, in the process of forming an aluminum film, to improve the burying characteristics of aluminum in the wiring groove and to minimize damage to capacitors. In order to meet these requirements, forming a liner made of titanium, niobium or a nitride of these metals (barrier metal) on the inner wall of the wiring groove is now being studied. There is a problem in this CMP process that the orientation of the aluminum film that has been deposited on the liner material is greatly altered by the liner material, and, according to the conventional polishing technique, the polishing rate of aluminum is very sensitive to the orientation of the aluminum film.
Namely, although a relatively high polishing rate of aluminum can be achieved when the aluminum film has a desirable orientation, the orientation of aluminum film may become undesirable depending on the structure of the liner, so that if the orientation of aluminum film becomes undesirable, the polishing rate of the aluminum film would be lowered to 1/10 as shown in FIG. 3 for instance.
It may be possible, if it is desired to achieve a practical polishing rate, to increase the polishing rate by raising the working pressure DF on the occasion of CMP from 300 gf/cm2 to 500 gf/cm2 for instance as indicated by the arrow in FIG. 3. In that case however, the planarity of the surface thus polished would be greatly deteriorated as shown in FIG. 4.
On the occasion of the CMP of a metal film, the surface of the metal film is generally ionized by an oxidant (a), so that a protective film made of a metal oxide or of a metal compound combined with an additive would be formed on the surface of the metal film (b) as shown in FIG. 6. Since this surface protective film is fragile in general as compared with pure metals, this surface protective film can be removed by polishing particles (c), and hence the polishing can be permitted to proceed by repeating these steps.
Therefore, keys to a high polishing rate are a rapid oxidation of the surface of metal film, the formation of an optimum protective film and the polishing power of the polishing particles.
In the conventional polishing technique, ammonium persulfate and hydrogen peroxide are generally employed. In this case, ordinary oxidation reactions represented by the following formulae are utilized.S2O82−+2e−→SO42−(E°=2.0 V)H2O2+2H++2e−→2H2O (E°=1.77 V)
However, since ammonium persulfate and hydrogen peroxide are relatively low in redox potential, i.e. E°=2.0 V or less, and hence, are insufficient in oxidizing power, an aluminum film having an orientation of (111) can be easily oxidized, whereas an aluminum film having an orientation of (110) or (100) can be hardly oxidized as shown in FIGS. 2A and 5A. Therefore, an aluminum film having a desirable orientation, i.e. an orientation of (111), can be polished at a high polishing rate. By contrast, in the case of an aluminum film having an undesirable orientation wherein crystal planes (110) and (100) are mixed together, the polishing rate thereof would be sharply deteriorated.